Awnings or canopies are often used for example outside restaurants or in private gardens to cover open areas and provide shelter for the people underneath them, thereby providing sun protection, rain protection or the like. A variety of covering apparatuses are known in the art. For example some canopies are provided on the side of a building, with folding arms which can be extended out from the building when the canopy is to be deployed. Other covering systems have a covering screen attached to moveable wires that are operated to extend or retract the screen.
It is desirable for canopies and awnings to be easily extended and retracted, whilst at the same time, the extended canopy must be strong enough to resist wind loading, etc. A major problem with all of these types of covering is providing sufficient tension in the screen so that the canopy does not sway unduly or flap in the wind. In the past, tension has typically been provided by means of springs so that as the screen is extended further, more tension is applied by the springs. However, the limit of tension which can be provided by springs is fairly low and varies with the length of the screen which has been extended. Also, springs lose their elasticity over time or if they are extended too far. This limit of tension has been a major factor in limiting the size (both width and length) of such covering systems. However, large covering systems are desirable for many applications, such as providing rain and sun protection over large patios, tennis courts or swimming pools and allowing them to be used or enjoyed in comfort.
The present inventor's earlier patent application, GB2421522, discloses a new type of covering system whereby a leading portion of a retractable screen is mounted to a number of longitudinal wires via clamping systems releaseably attached to the wires. In order to extend the screen, the clamping systems are released and pulled along the wires, thereby extending the screen with respect to the wires. When the screen has been deployed to the desired position, the clamping systems are clamped to the wires thereby maintaining the screen in the extended position under tension. Further tension can be applied by e.g. drawing back the trailing portion of the screen in the direction of retraction. Thus tension can be readily applied to the screen, and is limited only by the strength of the clamping system, the wires and the screen itself as opposed to springs as in the prior art. As such, this system enables large covers to be deployed under higher tensions.
It is disclosed in GB2421522 that the clamping systems and also the storage roller on which the screen is stored can be motorised in order to enable automatic deployment. However, the clamping systems will be subject to external forces such as wind, and other obstacles to movement such as debris on the tensile wires. The forces acting on each clamping unit may differ, thus causing them to move at different speeds despite having equal drive applied by the motors. Furthermore, the clamping systems can slip along wires (e.g. if they are wet or greasy), again, in a non-equal fashion. Also the motors may provide different drive forces, for example due to manufacturing tolerances or, variations in the power supply may cause one motor to pull momentarily stronger or to suffer a lull. A gust of wind on one side can cause a drag on one motor and a difference in elasticity in the fabric of the screen from one side to the other can cause different forces to be applied to motors on different sides of the screen. This can cause unbalanced pulling forces to be applied to the screen which can consequently be deployed and retracted in a lopsided fashion. Not only does this make the screen less effective, but it can damage the screen. These drawbacks will be particularly pronounced with larger screens where the clamping units have to travel greater distances and support heavier weights. In addition, in the case where no motor is provided for retraction, in order to retract the canopy it is necessary to provide for example a spring biasing mechanism. Where a large canopy is involved, a significant retraction force is required which can be difficult to provide.
When retracting the screen with unbalanced forces at either side of the screen, there is a further problem; with the unequal forces, the screen can become skewed and as it is retracted onto the storage roller, the screen may not roll up straight. In other words, as the screen is wound onto the storage roller, it may drift laterally one way or the other along the roller. In such cases, as the screen is rolled up, it is displaced gradually further and further along the storage roller in the axial direction. Also uneven pulling can lead to wrinkles forming in the screen. As these wrinkles are wound onto the screen, they become folds and creases in the fabric. Such wrinkles and creases create weaknesses in the fabric of the screen. Under the high tensions involved in such canopies, there is a greatly increased chance of the fabric tearing at such weaknesses. Once such distortions have manifested in the screen, they can persist indefinitely and become aggravated each time the awning is deployed and retracted.
Once the screen has been deployed, even when the clamps are clamped in place, external forces such as wind can still cause them to move, thus resulting in a loss in tension or an uneven tension in the screen. As the screen is raised significantly above the ground, wind passes both over and underneath the screen and the screen can easily catch the wind and experience high forces.